kanji radical 大 when used as a crown (top component)
Refers specifically to the kanji component 大 when it appears at the top of a character, such as in 天, 夫, or 失. This is a technical term used in kanji dictionaries and calligraphy.
The top part of 天 is called 大冠 (daikanmuri).
I look up the radical 大冠 in a kanji dictionary.
Compound of 大 (だい, 'big') and 冠 (かんむり, 'crown'), referring to the 大 shape when it sits at the top of a kanji like a crown.